selftests: drv-net: gro: increase the rcvbuf size

selftests: drv-net: gro: increase the rcvbuf size

The gro.py test (testing software GRO) is slightly flaky when
running against fbnic. We see one flake per roughly 20 runs in NIPA,
mostly in ipip.large, and always including some EAGAIN:

  # Shouldn't coalesce if exceed IP max pkt size: Test succeeded
  # Expected {65475 899 }, Total 2 packets
  # Received {65475 899 }, Total 2 packets.
  # Expected {64576 900 900 }, Total 3 packets
  # Received {64576 /home/virtme/testing/wt-24/tools/testing/selftests/drivers/net/gro: could not receive: Resource temporarily unavailable

The test sends 2 large frames (64k + change). Looks like the default
packet socket rcvbuf (~200kB) may not be large enough to hold them.
Bump the rcvbuf to 1MB.

Add a debug print showing socket statistics to make debugging this
issue easier in the future. Without the rcvbuf increase we see:

  # Shouldn't coalesce if exceed IP max pkt size: Test succeeded
  # Expected {65475 899 }, Total 2 packets
  # Received {65475 899 }, Total 2 packets.
  # Expected {64576 900 900 }, Total 3 packets
  # Received {64576 Socket stats: packets=7, drops=3
                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  # /home/virtme/testing/wt-24/tools/testing/selftests/drivers/net/gro: could not receive: Resource temporarily unavailable

Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20260107232557.2147760-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>